This invention relates to a semiconductor laser device which is used as a main structural component in an optical communication system.
As a progress of an optical communication technique, an application field of the optical communication technique has remarkably been spreading from a trunk line transmission system to an optical access network system of, for example, an optical subscriber system, an LAN (local area network), and a data link. Inasmuch as a lot of semiconductor laser devices are used in communication in such fields in various environments, it is requested that the semiconductor laser devices have a low cost and a resistance for various environments. To achieve this requirement, it is desired that a light emitting layer (namely, an active layer) and a buried layer of the semiconductor laser device are produced or grown by the use of an MOVPE (metal-organic vapor phase epitaxy) process which is also called an MOCVD (metal-organic chemical vapor deposition) process. By using the MOVPE process, the light emitting layer and the buried layer of the laser device can uniformly be grown on a substrate of a large area with a high controllability.
Under the circumstances, a conventional semiconductor laser device is reported by Y. Ohkura et al in Electronics Letters, 10th September 1992, Vol. 28, No. 19, pages 1844-1845, under the title of "LOW THRESHOLD FS-BH LASER ON p-InP SUBSTRATE GROWN BY ALL-MOCVD". The conventional semiconductor laser device has a buried structure on an InP (indium-phosphorus) substrate of a p-type and is produced by using the MOVPE process in all of crystal growth processes on manufacturing the laser device.
A similar semiconductor laser device is reported by Akihiko Oka et al in Technical Report of IEICE (The Institute of Electronics, Information and Communication Engineers), February 1993, OQE92-168, pages 13-18, under the title of "Low-threshold 1.3 .mu.m MQW laser array for optical interconnections". It is reported that the Oka et al semiconductor laser device exhibits threshold current of 2.6-3.0 mA, slope efficiency of 0.34-0.37 W/A at 25.degree. C. and threshold current of 8.1-9.2 mA, slope efficiency of 0.15-0.17 W/A at 80.degree. C.
However, inasmuch as any one of the abovementioned semiconductor laser devices has a current blocking structure (or a current confining structure) which confines a current in the active layer and which is constituted by a thyristor of a p-n-p-n structure, it is defective that the current blocking structure has a large capacitance within an optimum impurity concentration extent in which a low driving current is statically obtained. For example, the semiconductor laser device has a large capacitance of about 15 pF even when the semiconductor laser device not only has a narrow mesa stripe configura-tion in which a width of the current blocking structure is 30 .mu.m but also has a short cavity configuration in which a cavity (or resonator) length is 200 .mu.m.
When the semiconductor laser device has a large capacitance, a maximum modulation band is restricted to about 10 GHz on a fixed bias condition not less than an ordinary threshold value by restriction of CR time constant. Furthermore, on making the semiconductor laser device carry out zero-biased modulation in the manner described by T. Torikai et al in Technical Report of IEICE (The Institute of Electronics, Information and Communication Engineers), November 1993, OQE93-132, pages 43-48, under the title of "Adjustment-free MQW Laser Diodes for Fiber-Optic Subscriber Loop Application", an increase of jitter occurs on starting operation. This results in further limitation of the maximum modulation band. As a result, even modulation of 1 Gb/s is impossible. It is therefore necessary for the semiconductor laser device to have a capacitance which is not greater than 3.5 pF in order to realize a zero-biased 1 Gb/s modulation at a high speed.
On the other hand, still another semiconductor laser device of a small capacitance is reported by H. Wada et al in Electronics Letters, 19th Jan. 1989, Vol. 25, No. 2, pages 133-134, under the title of "1.55 .mu.m DFB LASERS WITH Fe-DOPED InP CURRENT LAYERS GROWN BY TWO-STEP MOVPE". The semiconductor laser device of a small capacitance is manufactured on an InP substrate of a p-type by using a semi-insulating InP material as a current blocking layer. The semiconductor laser device is realized which has a small capacitance not greater than 4 pF in a case where a cavity (or resonator) length thereof is 250 .mu.m.
However, inasmuch as the semiconductor laser device uses, as the current blocking structure, only the current blocking layer of the semi-insulating InP material which is doped with Fe (iron), the semiconductor laser device inevitably exhibits an increased threshold current of 30 mA at a room temperature. It is therefore impossible for the semiconductor laser device to operate a high-speed and zero-biased modulation which needs a low threshold current which is not greater than 5 mA.
The reasons why the semiconductor laser device exhibits an increased threshold current will now be described.
(1) Inasmuch as the current blocking layer of the semi-insulating InP material doped with Fe is unavoidably brought into contact with an InP layer of a p-type in the semiconductor laser device, diffusion of Fe into the InP layer of the p-type unavoidably occurs on manufacturing the semiconductor laser device. Due to such diffusion of Fe, the InP layer of the p-type inevitably has a decreased concentration of Fe. This results in an increase of a leakage current which flows outside the active layer.
(2) The current blocking layer of the semi-insulating InP material doped with Fe has no current blocking effect for holes. This also results in an increase of the leakage current which flows outside the active layer.
As described above, each of the above-mentioned semiconductor laser devices manufactured on the InP substrate of the p-type is incapable of easily realizing zero-biased 1 Gb/s modulation at a high speed.